The hybridogenetic hybrid R. esculenta arose through hybridization between R. ridibunda and R. lessonae. Usually hybrids are diploid. In some populations, however, they are triploid, with one genome from R. ridibunda and two genomes from R. lessonae (LLR-genotype). These triploids live with and mate with R. ridibunda. Thus, hybrids and R. ridibunda are sexual competitors. I studied the influence of the additional lessonae genome (dosage effect) on the morphology, biology and the electrophoretic pattern of two polymorphic proteins of triploid hybrids. The two lessonae genomes are not expressed according to a simple dose effect. Genetic information of the lessonae genome is obviously switched off because it would increase the dissimilarity between triploids and R. ridibunda, the parental species upon which reproduction of hybrids depends. Imprinting can account for the observed unusual pattern of inheritance.